It is already known to implement a direct digital synthesizer including a frequency latch, a phase accumulator, and a ROM based waveform generator. The ROM based waveform generator is used to drive a digital to analogue converter to produce the desired analogue output waveform. The phase accumulator is used to generate a sequence of digital binary values which are used to address the ROM to select stored digital values representing the waveform to be generated. The size of the ROM can be reduced by subjecting the digital values from the phase accumulator to a quantisation process to truncate the size of the digital words which address the ROM.
Such a quantisation process results in the introduction of periodic quantisation noise and it has already been proposed to add a dither signal to the output from the phase accumulator so that the digital values from the phase accumulator are added to random digital values from the dither generator. The result is to make the quantisation noise more random and thereby reduce the spurious quantisation components whilst accepting an increased total noise power.